1. Field of the Invention
The invention relates to a magnetic-inductive flowmeter for measuring the through-flow of a flowing medium, with a measuring line, a magnetic field generating device for generating a magnetic field—in particular, an alternating magnetic field—passing at least partially through the measuring line essentially perpendicularly to a longitudinal axis of the measuring line, and with at least two electrodes—in particular, in contact with the medium—for picking off a measuring voltage induced in the flowing medium.
2. Description of Related Art
The underlying principle of a magnetic-inductive flowmeter for measuring the through-flow of a flowing medium is attributable to Faraday, who back in the year 1832 proposed applying the principle of electrodynamic induction to measuring the flow rate of a flowing medium. According to Faraday's law of induction, in a flowing medium that entrains charge carriers and flows through a magnetic field, an electric field strength perpendicular to the direction of flow and perpendicular to the magnetic field is produced. Faraday's law of induction is used for magnetic-inductive flowmeters in that a magnetic field is generated by means of a magnetic field generating device, which usually has two magnetic coils to which current is applied, and is passed through a measuring line, the magnetic field generated having at least one component that extends perpendicularly to the direction of flow. Within the magnetic field, each volume element of the flowing medium that moves through the magnetic field and has a certain number of charge carriers makes a contribution to a measuring voltage that can be picked off by way of the electrodes with the field strength that is produced in this volume element.
For the determination of the through-flow of an electrically conductive medium, it is required that the measuring line is completely filled with the medium or, if it is not completely filled, at least that the degree of filling is known, so that the measured values can be corrected appropriately. Such correction values and the determination thereof are discussed, for example, in the German Patent DE 196 37 716 C1. The degree of filling of the measuring line can, for example, be determined capacitively, according to the patent specification DE 196 55 107 C2 and corresponding U.S. Pat. No. 6,092,428.
In this case, with the medium as a dielectric, the electrodes form a capacitor, the capacitance of which is measured. According to German Utility Model DE 20 2004 009 798 U1, the filling level of a liquid in the measuring conductor is determined by the transit time method. According to the German Patent Application DE 10 2009 045 274 A1, it is also made possible for measuring to be carried out for a partially filled measuring line by the electrode arranged above the measuring line having an electrically conductive continuation, which is located in the measuring line and during measuring operation is in connection with the medium. The laid-open German Patent Application DE 28 56 240 describes an alternating measurement of through-flow and degree of filling, for which at one time a constant magnetic field is generated and at another time an alternating magnetic field is generated.
It follows that, in the prior art, measuring principles that differ to some extent are combined in order to obtain findings concerning the filling level and the through-flow. To some extent, the values for the through-flow and the filling level are also obtained at different points in time.